These things are fundamentally connected. When the wealthy have too much power, they squeeze the middle and the poor too hard.
Unlike much else in life, the pool of money is a zero-sum game (though this is addressed in the paper, notably). When 3 Americans hold more wealth than 50% of the rest of us, that's a real problem. This historic and rising inequality leads nowhere good, and we are in existential crises which require that this be properly addressed.